1. Field of the Invention
This invention relates to a catalyst for the purification of the exhaust gas from a diesel engine. More particularly, it relates to a catalyst to be used in the form of a honeycomb catalyst for removing harmful components from the exhaust gas discharged by a diesel engine.
2. Description of the Prior Art
In recent years, the particulate substances emanating from diesel engines and comprising predominantly solid minute particles of carbon and liquid or solid minute particles of high molecular hydrocarbons (hereinafter referred to collectively as "particulates") have come to pose a problem from the standpoint of environmental hygiene.
The reason for this problem is that the particulates contain such harmful components as carcinogenic substances and these particulates mostly have diameters not exceeding 1 .mu.m and, therefore, are liable to be suspended in the air and inhaled into the human bodies via the respiratory organs. Thus, studies are under way in the direction of exacting the control of discharge of such particulates from diesel engines.
The methods which have been proposed heretofore for removing such particulates from the exhaust gas of a diesel engine include (1) a method which comprises filtering the exhaust gas of a diesel engine through a refractory gas filter (such as, for example, ceramic foam, wire mesh, metal foam, or mesh-closed type ceramic honeycomb) thereby sequestering the particulates, burning the carbonaceous particles accumulated on the filter by the use of heating means such as a burner or an electric heater, regenerating the filter, and putting the regenerated filter to reuse, (2) an improved version of the method of (1) which comprises causing a catalytic substance to be carried on the aforementioned filter thereby lessening the frequency of combustion and regeneration of the filter, and (3) a so-called filter method which comprises effecting combustion of the accumulated particles and purification of the exhaust gas under the conditions of discharge (gas composition and temperature) prevalent under normal running conditions of a diesel engine, for example.
These methods which involve the use of such filters as described above, however, are invariably intended to sequester solid fine carbonaceous particles in a high efficiency. Thus, they have the disadvantage that the filters are cracked by the local heat generated during the combustion of the accumulated particulates and the regeneration of the filters and, further, the filters are clogged and the catalysts are impaired in activity because of the accumulation of ash components (such as, for example, calcium oxide, zinc oxide, and phosphorus pentoxide) originating in the engine oil and sequestered in conjunction with fine carbonaceous particles. Further, the apparatuses which embody the filter-type method for the purification of the exhaust gas have the disadvantage that their operation entails pressure loss. None of the filter-type methods heretofore proposed for the purification of the exhaust gas proves to be fully satisfactory from the practical point of view.
The improvements (such as, for example, an increase in the pressure used for fuel injection and control of the timing of fuel injection) introduced in recent years to the diesel engines have served the purpose of decreasing the amount of particulates discharged from the diesel engines. At the same time, in the particulates discharged from the diesel engines embodying these improvements, the proportion of the fraction predominantly comprising liquid high molecular hydrocarbons and exhibiting solubility in organic solvents (hereinafter referred to as "SOF (soluble organic fraction)") has increased. The exhaust gas emanating from the improved diesel engines differs in behavior from the exhaust gas discharged from the conventional diesel engines. In the purification of the exhaust gas of such a different behavior, therefore, the removal of the SOF predominantly containing harmful components such as carcinogenic substances poses itself an important problem.
As a catalyst for the removal of the SOF in the exhaust gas from the diesel engine, the feasibility of an open type honeycomb catalyst possessing through holes disposed parallelly to the flow of gas has been studied and reported besides the aforementioned filter-type method (SAE Paper, 810263).
This open type honeycomb filter, however, is identical in construction to the honeycomb catalyst used for the purification of the exhaust gas from the ordinary gasoline engine and can be hardly called fit for the purification of the exhaust gas from the diesel engine. None of the catalysts heretofore proposed has proved to be fit for the purification of the exhaust gas possessing the behavior described above.
An object of this invention, therefore, is to provide a novel catalyst for the purification of the exhaust gas from a diesel engine.
Another object of this invention, directed to the purification of the exhaust gas from a diesel engine which has a small content of particulates in the exhaust gas and a large SOF content in the particules, is to provide a fully practicable catalyst capable of effecting the purification with high efficiency and subserving the function satisfactorily without entailing any notable problem in a vehicle operated with a diesel engine.